In recent years, by using an image formation method of an electrophotography system employing an electrostatic image developing toner, formation of a full color image (a full color print) has been performed in addition to formation of a monochrome image (a monochrome print) which has been used mainly for creation of a document etc.
In the image formation method of the electrophotography system employing such a toner for electrostatic image development, color toner particles are used to form a color image. Examples of the color toner particles are a yellow toner particle, a magenta toner particle and a cyan toner particle, each of these color toner particles containing a binder resin and colorant for each color. By superposing the toner images formed by each of these color toner particles, a color tone is adjusted and, thereby, a full color image is formed.
In order to form a full color image especially used for a catalog, an advertisement, and the like, it is required that an original image should be reproduced faithfully. As a consequence, a good color reproduction property is required for color toner particles in order to form a full color image having a desired color tone.
Application of various organic pigments and dyes to a colorant for a color toner particle has been examined. Examples of colorants used for a magenta toner particle are: an azo lake pigment, an anthraquinone dye, a quinacridone pigment, a rhodamine dye and its lake pigment (for example, refer to patent documents 1-6.)
However, there are several problems to use toner particles containing these colorants. Specifically, it is hard to obtain an intermediate color (half-tone) image having a high enough quality especially by using a color toner particle incorporating a low molecular weight dye such as a rhodamine dye (a rhodamine dye compound). Further, when the color toner particle is kept for a long time at an ordinary temperature, the toner particle tends to be aggregated to become a granular particle and to result in failing to obtain sufficient fixability of an image.
In the color toner particle which incorporates a dye of low molecular weight as a colorant, the causes which produce the above-mentioned problem are thought to be as follows. One of the causes may be a decrease of a glass transition temperature of the color toner particle incorporating the dye of low molecular weight due to that fact that this dye has a high compatibility with a resin. Another cause may be a decrease of thermal energy which should be used for fixing of the toner, because this dye may be sublimated during the thermal fixing of the image. This means that a part of thermal energy will be consumed by sublimation of this dye and sufficient thermal energy is not provided for fixing.
Patent Document 1: Unexamined published Japanese patent application (hereafter it is called as JP-A) No. 5-297633
Patent Document 2: JP-A No. 2005-298458
Patent Document 3: JP-A No. 10-226673
Patent Document 4: JP-A No. 11-335339
Patent Document 5: JP-A No. 6-49008
Patent Document 6: Japanese translation of PCT international application No. 2005-533764